feat(distribution): 分站可下调收号比例且不超过主站上限

让分销站能按自身能力下调收号比例扩大利润，同时把主站回收比例作为硬边界，避免分站把比例加得比主站更高。

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/internal/model/distribution.go

@@ -17,6 +17,7 @@ type DistributionSite struct {
 	FooterInfoJSON        string    `gorm:"type:text" json:"footer_info_json"`
 	DefaultSupportGroupID *uint64   `gorm:"index" json:"default_support_group_id,omitempty"`
 	ListingQrcodeURL      string    `gorm:"size:512;not null;default:''" json:"listing_qrcode_url"`
+	ReceiveRatioDelta     float64   `gorm:"type:decimal(10,2);not null;default:0" json:"receive_ratio_delta"`
 	TaxEnabled            bool      `gorm:"not null;default:false" json:"tax_enabled"`
 	TaxRateBPS            int64     `gorm:"not null;default:100" json:"tax_rate_bps"`
 	SettlementHoldDays    int       `gorm:"not null;default:7" json:"settlement_hold_days"`